There are known transport mechanisms for transporting a tape between two reels on which the opposite ends of the tape are secured. The reel from which the tape is unwinding is generally referred to as a supply reel and the reel on which it is being wound is generally referred to as a takeup reel. When the tape is transported longitudinally in a forward direction, the effective diameter of the tape pack wound on the supply reel is decreasing while it is increasing on the takeup reel.
For example, in magnetic recording/reproducing devices a magnetic tape is transported past one or more electromagnetic transducers which may record an information signal on the tape or reproduce an information signal which has been previously recorded on the tape. During the recording and reproducing processes the longitudinal tape speed is accurately controlled, generally by a capstan servo, which drives the tape at a selected constant speed past the transducers, as it is well known in the art of magnetic tape recording and reproduction. As the tape is being wound progressively on the takeup reel, it is important to control the tape tension to avoid winding of the tape pack too tightly or too loosely. Winding the tape too tightly consumes excess power and may damage the integrity of the tape pack, or the tape itself, thereby jeopardizing the recorded information. On the other hand, when the tape is wound too loosely it may also adversely affect the integrity of the tape pack, or cause sliding of the tape off the reel or other parts of the transport, thereby loosing valuable information signal. To avoid the foregoing problems, magnetic tape recording/reproducing devices are known to utilize tension arms which are arranged in contact with the tape along the tape path. The rotation of the takeup and supply reels is controlled to maintain the tension arms in a desired position during the recording and playback operation, which position corresponds to a desired tape tension.
To obtain the foregoing, a position servo, generally referred to as the reel servo, is utilized in a well known manner. Generally, the reel servo receives a position command signal corresponding to a desired position of the tension arm, and a negative feedback position signal, corresponding to its actual position. The servo provides an error signal corresponding to a difference between the received signals and it applies a control signal to accelerate or decelerate the reel until the position error signal becomes zero. By rotating the reel faster or slower, the position of the tension arm changes, thereby changing the tape tension to tighten or loosen the tension within the tape pack on the reel, until the desired tape tension is obtained. Thus the position of the tension arm is servoed by controlling the rotation of the reel, while the longitudinal tape speed remains constant.
However, as the tape pack diameter changes during operation, it is often required to change the desired tape tension in accordance with a desired tape tension profile. Tape tension profiling generally facilitates the use of lower tape tension values, as it is for example desirable in applications where low power consumption is an important consideration. There are known tape recording/reproducing or tape conditioning devices which change the position command signal for the tension arm as the diameter of the tape pack on the takeup reel increases or decreases, to obtain optimum tension within the pack during the entire operation. These known devices utilize tape pack diameter sensors for measuring the changing diameter of the tape pack wound on the takeup reel. In response to the measured tape pack diameter, the known tape tension profiler may adjust the desired tape tension by changing the tension arm position, or the spring tension applied to the tension arm, or by changing the takeup reel torque where a tension arm is not used. For example, to change the tension arm position, the known tape tension profiler would change the tension arm position command signal to the takeup reel servo, which in turn rotates the takeup reel, until the tension arm is repositioned to a desired position. Thus a new position of the tension arm corresponds to a new desired tension of the tape wound on the reel which, in turn, is obtained as a function of the measured tape pack diameter. It is generally referred to the above-indicated function as a tape tension profile. For example, some tape conditioner manufacturers recommend tape tension profile values which have higher tape tensions closer to the beginning and end of the tape pack, that is near to minimum and maximum tape pack diameters, and lower tensions at intermediate diameters.
In recording/reproducing applications where it is desirable to simplify the tape transport because of space or weight restrictions, such as in aircraft or satellites, or where there is no immediate access to the apparatus during the recording operation, such as in remotely controlled devices, proper control of tape tension is even more important because usually there is no backup recorder, and valuable data may be irretrievably lost due to poor tape pack integrity or tape damage.